1. Field of the Invention
The invention relates to an assembly for use in seismic surveys of the sea bed from an oceangoing vessel, wherein groups of seismic signal transmitters are towed one behind the other and/or beside each other through the sea, the transmitters being supported and suspended from float buoys.
2. Description of the Prior Art
In seismic explorations of the sea bed, it is necessary to produce data characteristic of the individual earth formations. This is done by utilizing energy sources which emit pressure pulses, which propagate downwardly to the geological stratifications in the sea bed and are reflected back to a receiver and recorded. The pressure pulse transmitters, which normally are air guns or gas guns, are towed behind a vessel, being suspended from float buoys and thus positioned a certain distance beneath the surface of the sea. For each firing, an acoustic cable registers a reflection pattern. By continuous measurement of this reflection pattern, a picture of the geological structure, showing strata, faults, etc., is obtained. The air or gas guns work by releasing a certain volume of air which oscillates approximately as a slowly decaying wave, the period of oscillation being dependent upon the volume of air released. In such pulse reflection from geological formations, it is desirable that the reflection wave obtained have as "peaked" a shape as possible, so that scattering effects may be avoided. Thus, the goal is to obtain as pure a transmitter signal as possible. To obtain this, a plurality of mutually tuned air guns generating pulses simultaneously are utilized, being so tuned that pulse train reverberations are compensated/counteracted while the first half period of oscillation is reinforced. The guns are positioned within a restricted area and may be looked upon as a single transmitter which, owing to the cooperative action of the guns, provides a sharp or peaked pulse. Such transmitter sources, which consist of several guns positioned within a restricted area, may be termed point sources. In seismic surveying, several such restricted areas or point sources are streamed out behind a vessel, arranged in an array with one source behind another and/or beside another and towed through the sea. A system which operates according to this principle is described, e.g., in Norwegian Pat. No. 138.922, which corresponds to U.S. Pat. No. 3,953,826.
To obtain a good reflection signal which is as sharply defined as possible, however, it is important that the transmitters within each restricted area or in each point source are positioned at substantially the same depth. With the conventional streamer systems in use at present, each separate gun in the restricted area is suspended from its own float buoy, and the individual guns may very easily lie at different depths in the water, depending on wave movement. This is unfavourable for obtaining correct results.
A further drawback of such tow systems is the high resistance of the tow to propulsion in the sea. Moreover, the position of the tow point between the vessel and the point source will be of decisive importance both for the individual gun locations and for the resistance to movement through the sea.